The following invention relates to antennas, and in particular, vehicle antennas designed for use with citizen band radios.
Citizen band (CB) radio generally refers to short-distance broadcast and receiving equipment operating in the broadcast frequency between 26.9 and 27.7 megahertz. CB radio waves in this frequency range are typically divided into forty discreet broadcast bands, each having a wavelength close to to 36 meters. The optimal antenna length for radiating and receiving such waves is thus about 36 feet.
Efficient CB antennas suitable for vehicles may be constructed as quarter or eighth wavelength antennas. For example, the so-called "108 whip", having a 108 inch length, is a quarter wavelength CB antenna. Antennas of this type may be further equipped with an adjustable tip which is variable to change the total length of the antenna for achieving optimal tuning at a desired CB frequency band.
Alternatively, CB antennas suitable for vehicle mounting may be constructed by wrapping an approximately 36 foot length of low-resistance wire conductor about a short antenna rod. Optimally, the wire conductor is helically wound around the rod, although overlapping coiled windings may also be employed. An antenna of this type is equivalent to an L-C circuit having an induction coil and capacitor in parallel. The antenna may be tuned by varying the inductance of the antenna, e.g., by changing the length of the coil or varying the number of coil windings.
The performance of the CB antenna can be further enhanced by optimal impedance matching between antenna and CB radio equipment. From the above, it is evident that the impedance of a helically-wound antenna may also be varied by changing the length of the coil and/or the number of helical windings on the antenna.
It is an object of the present invention to provide a tunable helically-wound radio antenna designed for mounting to vehicles, boats or the like.
Specifically, it is an object of the invention to provide a helical whip-type antenna which can be tuned to CB radio bands.
It is another object of the invention to provide an antenna having a selectively variable impedance.
Yet another object of the invention is to provide a helically-wound antenna which is lightweight, durable, and inexpensive in manufacture.
The antenna of the present invention comprises an antenna rod adapted for mounting to a vehicle or the like, and a low-resistance wire conductor wound helically about the rod. The conductor windings in the tip region of the rod are selectively movable to vary the frequency response and the inductive impedance of the antenna.